1. Field of the Invention
Exemplary aspects of the present invention relate to a method and apparatus for image forming, and more particularly to a method and an apparatus for toner image forming capable of detecting a reference signal used for a lighting control.
2. Description of the Related Art
A related art image forming apparatus such as a laser printer, a digital copying machine, a facsimile, etc., has employed an optical beam scanning method for an image writing. According to the optical beam scanning method, a lighting (light emission) of a laser diode (LD) outputting a plurality of optical beams is controlled by image data, and a rotation polygon mirror deflects the plurality of optical beams to scan periodically in a main scanning direction. Thereby the optical beams irradiate a photoconductor moving towards in a sub-scanning direction so as to write an image on the photoconductor by each line.
When the image is written on the photoconductor by the optical beam scanning method, the photoconductor has an image writing start position thereon to begin the image writing. Since this image writing position needs to remain constant for each scanning line, a synchronous detection sensor is used to detect that the optical beams are disposed outside an image region of the photoconductor in an image writing start position side. The synchronous detection sensor detects a passage of the optical beams scanned in the main scanning direction (also referred to as a main scanning line) by the rotation polygon mirror, and instructs a lighting timing of the LD for each main scanning line with a synchronous detection signal generated thereby as a reference so as to control the image writing start position to be constant.
Regarding a color image formed by a related art color image forming apparatus, in general, a number of the photoconductors to be used is substantially equal to a number of color components. The photoconductors are scanned by the optical beams of respective color components, and images of different colors are superimposed by a transfer process so as to form a full color image. This formation of the full color image is referred to as a tandem system, and has been widely used.
As the tandem system exposes the photoconductors of respective color components to the optical beams, the image writing is controlled for each color component based on the synchronous detection signals generated by the synchronous detection sensors.
Therefore, the related art color image forming apparatus employing the tandem system generally superimposes the images of four colors, i.e., yellow, magenta, cyan, and black, formed on respective photoconductors so as to form the full color image. Thereby, the synchronous detection signals for the four colors are generated by using four different synchronous detection sensors.
In addition to using the four synchronous detection sensors with respect to the optical beams of respective colors, one example has attempted to use two synchronous detection sensors in another related art color image forming apparatus. Each synchronous detection sensor is commonly used for two color components.
According to this example of using the two synchronous detection sensors, one polygon mirror capable of scanning the four color components is employed. The four optical beams for the four color components are divided into two groups, for example, black and cyan, and magenta and yellow. The one polygon mirror has a plurality of mirror faces into which the optical beams are entered with respect to each group. The two optical beams of different color components in each group are detected by one of the two synchronous detection sensors. Each of the two synchronous detection sensors outputs the synchronous detection signals of the two optical beams which can be separated based on a time period by shifting a detection timing of each optical beam.